1. Field of the Invention
This invention relates to focus detecting devices.
2. Description of the Prior Art
In a known, TTL in-focus detection device, light passing through a phototaking optical system is focused on a focus-detecting photoelectric transducer element located at a point conjugate to the film plane. In response, focus detecting output signals are produced by which the in-focus condition of the phototaking optical system is detected. Many proposals relating to this device have been made. When such in-focus detecting device is to be built into a single lens reflex camera or the like, it is necessary that it be arranged in a limited space within the black box of the camera. Hence, the device must be made quite small in size because of the fact that a portion of the object image is formed on a light-receiving surface of the focus-detecting photoelectric transducer element. In such an arrangement, dust and finely divided foreign particles floating in the interior of the black box may be deposited on the light receiving surface of the in-focus detecting photoelectric transducer element and may adversely influence the focus detecting signals formed by the same element. Counter-measure to this become a problem. That is, for example, if a pair of photosensitive element arrays each consisting of a plurality of minute photoelectric transducer elements are used in detecting an in-focus condition and two images of the same object formed on the respective arrays coincide in position with each other, so that when dust and foreign particles are deposited on a portion of the light incident surface of the photosensitive element arrays, the coincidence signal representing the in-focus state is disturbed. This causes faulty operation. Also, the so-called image sharpness degree detecting type device similarly suffers from the deposition of dust and foreign particles on the image receiving surface of a photosensitive element because this influences the focusing state of the image and, in effect, does nothing but provide an erroneous detection.
What has been discussed above is valid not only in the TTL type but also in the non TTL type focus detecting device, and further not only in the passive type but also in the active type devices. For example, even in a sonar type device utilizing supersonic waves, there also is a high possibility of the existence of dust and foreign particles on the sensing surface of a supersonic wave sensor to effect interference of the waves, thus causing the device to malfunction.